Node ``mriqc.interfaces.functional.Spikes`` is a memory hog

[oesteban]

This node in the workflow https://github.com/nipreps/mriqc/blob/d5b13cb56b6de1493ad91979f0e1e48ebb6708db/mriqc/workflows/functional/output.py#L95-L100 often causes memory errors.

In a quick review, I believe casting here https://github.com/nipreps/mriqc/blob/d5b13cb56b6de1493ad91979f0e1e48ebb6708db/mriqc/interfaces/functional.py#L229 to float32 would be safe and have zero effects on the interface performance.